1. Field of the Invention
The present invention relates to the preparation of polyvinylidene chloride (PVDC)-coated polyester films, the adhesiveness of which is improved by an adhesion primer layer, and the PVDC-coated oriented composite polyester films thus produced.
2. Description of the Prior Art
Oriented polyester films are a substrate of choice in the packaging field because of their desirable mechanical properties and their transparency. However, it is known to the art that, because of their chemical inertness, their crystallinity or their relative permeability to gases and water vapor, it has proved essential to provide these materials with a coating which is capable of obviating all or some of these defects and, at the same time, imparting new properties thereto. Thus, the application of a polyvinylidene chloride coating on at least one face surface of oriented polyester films to provide packaging films having, on the one hand, excellent properties of impermeability to gases (especially to carbon dioxide and to oxygen) and to water vapor and, on the other hand, an excellent suitability for sealing by heat (heat-sealability) has been proposed to this art. To this end, aqueous dispersions or emulsions of polyvinylidene chloride or of copolymers of vinylidene chloride with a small quantity of one or more unsaturated monomers are employed. These dispersions are applied by the usual coating processes (for example by immersion, spraying or roller coating) onto the polyester film, after orientation by drawing (post-coating). Compare French Patents Nos. 1,255,353 and 1,478,603. However, the coatings thus obtained have the disadvantage of not adhering very well to the polyester support film. In order to improve the bonding between the polyester film and the PVDC coating, it has been proposed to first apply an anchor layer or adhesion primer layer to the surface of the film destined to receive the coating. Various polymeric materials have been proposed in order to achieve this objective. Organic solutions of polyurethanes have to date proven to be most advantageous with regard to improving the adhesiveness of PVDC to polyesters. However, the use of organic solvents presents many disadvantages from industrial (for example from the point of view of hygiene and safety) and economic (investments necessitated by the recovery of solvents) standpoints. Therefore, the film industry is attempting to avoid use of adhesion primers of this type. For example, improving the adhesiveness of PVDC to polyester films using aqueous emulsions of mixtures of PVDC and a water-dispersible copolyester having a plurality of hydroxysulfonyl-(--SO.sub.3 H)-group-containing recurring units containing hydroxysulfonyl moieties or the alkali or alkaline earth metal salts thereof (hereinafter referred to as "oxysulfonyl groups") has been proposed in Japanese Patent Application published under No. 58/147,354. While providing beneficial results, this solution is not free from disadvantages. Firstly, it entails the use of copolyesters having a high content of oxysulfonyl group-containing recurring units in order to ensure the water-dispersibility of the copolyesters. Moreover, the PVDC-coating layer thus obtained is rendered more sensitive to moisture because of the presence of a hydrophilic copolyester in the surface coating. Serious need thus remains in the film industry for means which permit the adhesiveness of PVDC to polyester films to be improved, but without adversely affecting the homogeneity of the coating and without requiring the use of organic solutions of adhesion primers.